Radio receiver



April 28, 1-936. D. GRIMES 2,038,730

RADIO RECEIVER Filed' July 26, .1932 2 Sheets-Sheet 1 70 UUZ/ZATIOA M549 In? jg X K) a Q Q INVENTOR g N DAVID GRIMES 5% Q BY 6 R N,\

ATTORNEY Patented Apr. 28, 1936 UNITED STATES PATENT OFFICE RADIO RECEIVER Application July 26, 1932, Serial No. 624,708

Claims. (Cl. 250-20) My present invention relates to high frequency signalling systems, and more particularly relates to radio and other high frequency receiving, amplifying and repeating systems, and circuits therefor.

The utilization of an electron discharge tube amplifier for amplifying two separate frequencies simultaneously without cross interference has long been known by the terms of reflex or duplex. Such circuits employing two or more, duplex tubes have been successfully operated in the past by the inverse duplex system. In my United States Patents 1,517,057 and 1,517,058, both patented November 25, 1924, there are described and claimed electron discharge tube ampliflcation and reception systems wherein the amplifier tubes are arranged to effect both radio frequency and audio frequency amplification simultaneously, but in which the order of ampliflcation differs from the previously known reflex methods in that the audio frequency amplification is accomplished in the inverse order as compared with the order, or sequence, in which the tubes successfully function toeffect radio frequency amplification. Such a system will be hereinafter identified by the term inverse duplex.

No serious diificulties have been encountered in operation with an inverse duplex receiver where normal grid bias has been employed and the instantaneous grid swing of the tubes did not run either into the positive region or the cut-off region of the tube characteristics. Inverse duplex receiving sets were quite satisfactory at the time of their introduction because of moderate power broadcasting stations, and in virtue of moderate gains employed in amplifiers at the time. However, with the increase in modern broadcasting power, such as has been experienced with 50 kilowatt stations and the like and. improvements in the art which have made possible greater amplification in receivers, an instantaneous grid swing of considerable magnitude results in the grid entering the positive region or the cut-off region of the tube characteristic. As a consequence inverse duplex receiver sets, designed according to my aforementioned patents and operated at the present time, have been productive of much distortion and modulation howl.

Now, I have discovered a method of, and devised arrangements for, successfully operating an inverse duplex receiver at the present time without producing the aforementioned undesirable reproduction difficulties. Briefly, the aforementioned distortion and modulation howl, has been effectively eliminated by the employment of remote cut-off tubes used in combination with automatic gain control circuits. The elimination or reduction of the undesirable conditions already referred to is accomplished in the following manner:

When a duplex circuit is tuned to a strong local station the normal bias necessary for maximum sensitivity is automatically increased to effect a reduced amplification as well as to permit greater instantaneous grid swing without the grid entering the positive region. Meanwhile, in virtue of the remote cut-ofl tube, the increased instantaneous grid swings do not enter the cut-off region so that distortion and modulation howl do not arise from this source. It is thus seen that the normal grid bias which is near the plus region and which is necessary for maximum gain, is automatically backed 01f and adjusted to accommodate the greater swings of stronger stations. The automatic volume control feature alone does not solve the problem as the increased bias on other than remote cut-off tubes would run the signal into the cut-off region Where the same difliculty arises.

Hence, it may be stated that it is one of the main objects of the present invention to provide a radio receiver wherein the combination of automatic gain control and remote cut-off tubes admirably lends itself to the difiiculties in duplexing encountered with the great amplification of modern amplifiers and the increased strength of 7 modern broadcast receivers.

Another important object of my present invention is to provide a radio receiving system of the reflex type which is so constructed that adjustment of negative grid bias of the amplifier tubes can be employed for control of the intensity level of the receiver output, it being especially noted that such intensity adjustment cannot be per-. formed in known reflex radio receivers of the prior art.

Another object of the present invention is to provide a radio receiver of the reflex type with an automatic gain control circuit in order to prevent overload howl and distortion effects which howl and distortion efiects are customarily noticeable on strong signals with known reflex receiving systems of the prior art.

And still another object of my present invention is to provide in a reflex radio receiver an automatic gain control circuit, the constants and characteristics of the receiver and volume control circuit being so chosen that the characteristic relating receiver output intensity and receiver input intensity is substantially uniform.

And still other objects of the present invention are toimprove generally the efficiency of reflex radio receivers, and to particularly provide a reflex radio receiver which is not only economical, durable and reliable in operation, but capable of distortionless, faithful and uniform reproduction,

and is, additionally, economically manufactured I and installed. 7

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into 7 effect.

In the drawings,

Fig. 1 diagrammatically shows a radio receiving system embodying the presentinventiocn,

Fig. 2 diagrammatically showsa modified form of the invention, a V

Fig. 3 graphically demonstrates the operation of a reflex radio receiver embodying the present invention. 7 V 1 7 Referring now to the accompanying drawings wherein like reference characters in the different figures indicate similar elements, the present invention contemplates using the same electron discharge tube amplifiers for efi'ecting both radio frequency amplification and audio frequency amplification, but instead of passing the audio frequency currents through the amplifier tubes in the same sequence as the radio frequency currents thus unequally loading the tubes, the sequences or elimination: of tube distortion.

, As clearly explained in my aforementioned V U. S. Patent 1,517,057 the elimination or reduction of the tendency to sustainhigh frequency oscillations is accomplished in the circuits of the inverse duplex receiver by reason of the fact that the output of the detector is impressed on the input circuit of the last high frequency amplifier tube instead ofone of the preceding high frequency amplifier tubes. Accordingly, any high frequency oscillations which may be transmitted from the output of the detector to the input of the last high frequency amplifier tube through the medium of the intervening low frequency'path are amplified by only one tube.

In Fig. 1 there is shown a circuit diagram of an inverse duplex receiver and amplifier comprising two amplifier tubes l and 2 and a. detector tube 3. Each of the amplifier tubes l and 2 is of the type known as an exponential, or variable mu, tube. The detector tube 3 is shown as a screen grid tube of the usual and well known type, and need not be described in any further detail. With regard to theconstruction of the variable mu amplifier tubes land 2 it is merely necessary for the purposes of the present disclosure, to point out that these tubes are characterized by the fact that they have a remote cut-off grid voltage-plate current characteristic. The characteristics and nature of variable mu tubes, as Well as various specific constructions of variable mu screen grid tubes, have been described by Ballantine and Snow in the Proceedings of the Institute of Radio Engineers for December, 1930.

However, independently of the aforementioned publication, those skilled in the prior art at the present time are Well aware of characteristics and construction of variable mu tubes to clearly understand the nature of the tubes l and 2 employed in the present invention. For the purposes of illustration, then, each of tubes l and 2 has been shown to comprise a control grid, an anode, a cathode of the indirectly heated type (that is to say an equi-potential cathode), and a. screen grid 4 arranged in such a manner, this being symbolically represented as shown in Fig. 1 by the frustrum of a cone, that the tube has a. plate currentgrid voltage characteristic with a remote negative cut-off point. Each of the screen grids 4 is arranged to have positive potential applied thereto from a source S, (not shown), it'being noted that the source B supplies potential for the anodes of, the tubes l, 2 and 3, as well as the utilization means employed to receive the low frequency output of the receiver.

. In Fig. 1, the amplifier tubes I and 2 are shown utilized for both radio frequency and audio frequency amplification, but it is to be clearly understood that the present invention is not limited to this arrangement, since the tubes l and 2 may be disposed in the intermediate frequency amplifier circuits of a superheterodyne receiver, whereby the audio frequency output of the second detector is to be amplified inthese intermediate frequency stages. Additionally, it is to be clearly understood that the tubes I and 2 may be disposed in the radio frequency amplifier stages of a'superheterodyne receiver, and that the output of the tube-3 impresses intermediate frequency energy upon the tubes I and 2 for amplification at intermediate frequency. a

In otherwords, the present invention is not limited to the arrangement shown in Fig. 1, nor is it limited to the number of amplifier tubes shown, nor'is it limited to a reflex receiver of. the inverse type, since it is equally well adapted to the conventional reflex receiver. Furthermore, in some cases, it may be desirable to utilize the tubes I. and 2 for amplifying radio frequencies, intermediate frequencies and audio frequencies, as in the case of a superheterodyne receiver wherein the tubes l and 2 would be disposed in theradio frequency stages and the outputs of the first and? second detectors have been impressed upon the radio frequency stages for amplification at three different frequencies. A number of other arrangements may also be resorted todepending upon the number of amplifier tubes and theamount of high frequency and low frequency amplification desired.

The conductors 5 may extend to a signal c01 lecting system such as a' grounded antenna circuit, or other terminals of a loop antenna, or they may connect with a metallic transmission line as in the case of so-called carrier current, or wired radio telephone systems. These various en ergy sources have been generically designated as a source of signal energy. The variable conquency currentswhile substantially blocking audio frequency current. Bias for the grid of tube is secured by the well known arrangement shown in the cathode lead of tube I, such arrangement comprising a resistor shunted by a by-pass condenser, the arrangement being generally designated by the numeral 8. The low potential end of the biasing resistor is grounded.

The cathode lead of the tube 2 includes a similar grid biasing arrangement 8', the low potential end of the biasing resistor being grounded, and an audio frequency blocking condenser I being disposed in a similar manner as the condenser I. Each anode of tubes I and 2 is connected, through a radio frequency by-pass condenser 9, to its respective cathode as shown. Coupling transformers M2 and M3 are used respectively for coupling the output circuit of tube I to the tuned input circuit of tube 2 (it being noted that a variable condenser 6' is used to tune the input circuit of tube 2), and the output circuit of tube 2 to the tuned input circuit of the detector tube 3.

The variable condenser I0 is connected across the secondary of the coupling transformer M3. and any well known type of mechanical uni-control arrangement II, shown in dotted lines, is used for simultaneously varying the variable condensers 6, 6' and I0. The grid circuit of the detector tube 3 includes the well known type of grid rectification arrangement, designated by the numeral I2, and the anode of tube 3 is connected to the grounded side of the cathode of the tube through a radio frequency by-pass condenser I3.

As explained heretofore, the low frequency energy output of the detector tube 3 is amplified in the last high frequency amplifier stage of the receiver. Hence, the low frequency output of detector tube 3 is impressed upon the primary ll of an audio frequency transformer I through a path which includes the lead I6, the primary I4, and a lead I I which is adjustably connected to the source B so that a proper positive potential can be applied to the anode of tube 3, it being noted that the negative side of the source B is grounded. The low frequency energy impressed across the primary I4 is transferred to the input circuit of the amplifier tube 2 through a path which includes the lead I9, the secondary I8 and a lead 20 adjustably connected to resistor 2|, it being pointed out that the reason for the adjustable connection of the lead 20 is explained in detail hereinafter.

It will be noted at this point, however, that the resistor 2| is connected by a lead 22 to ground, and hence it will be seen that the energy trans ferred to the secondary I8 is impressed between the grid and cathode of tube 2. The amplified low frequency energy is then transferred from the output of tube 2 to the input of the next preceding amplifier tube I by impressing the amplified low frequency energy upon the primary circuit of a second audio frequency transformer 23 through a path which includes the lead 24, the primary 25, the lead 26, the lead 21, the source B, and then through ground back to the grounded side of the cathode of tube 2. The energy transferred through the audio frequency transformer 23 is then impressed between the grid and cathode of tube I through a path which includes the lead 28, the secondary 29, the lead 3!! which is connected to one end of'the resistor 2|, the resistor 2|, the lead 22, and then through the grounded side of the cathode of tube 3 back through ground to the grounded side of the cathode of tube I.

The amplified low frequency output of tube I may, then, be utilized in any well known manner,

and this utilization path includes the lead 3|, the utilization means, the lead' 32, the lead 21, the source B and through ground to the grounded side of the cathode of tube I. The utilization means, although generically designated may be a power amplifier tube succeeded by a loud speaker or any other type of reproducer, or the reproducer may be directly connected for receiving the amplified energy of the tube I without the use of a power amplifier tube.

For regulating, or controlling, the amplification of the tubes I and 2 there is provided an automatic gain control circuit which comprises a rectifier, such as a diode 33, the cathode of which is connected by a lead 34 to the high potential side of the secondary of the coupling transformer M3, while the anode is connected to the grounded side of the cathode of tube 3 through a path which includes the lead 35, the resistor 2| and the lead 22. A resistor 2| is' shunted by a radio frequency by-pass condenser 36, and it has already been explained that the high po tential end of the resistor 2| is connected by the lead 30 to the grid circuit of tube I, and that an adjustable lead 23, connected by any desired point of resistor 2|, is connected to the grid circuit of tube 2. By proper setting of the adjustable tap of lead 23 on the resistor 2| it is possible to secure a characteristic as shown by the full line in Fig. 3 which graphically relates the receiver output to the receiver input. It will be noted that the full line characteristic shown in Fig. 3 is substantially uniform over a substantial range of the receiver input.

The receiving system shown in Fig. 1 operates in substantially the same manner, as far as the duplex action thereof is concerned, as the inverse duplex receiver disclosed in my aforementioned U. S. patents. The signal energy is impressed through the coupling M1 upon the input circuit of the first stage of tuned radio frequency amplication. The unit-control tuner II is adjusted to that setting which will render the radio frequency stages resonant to the desired signal frequency. The desired signal frequency energy is amplified in the first stage of radio frequency, the amplified output of tube I is then impressed upon the tuned input of the radio frequency stage including tube 2. The amplified radio frequency output of tube 2 is then detected in the detector stage, and the detected output of tube 3 is transferred, through the audio frequency transformer I5, to the inputof tube 2, now func: tioning as an audio frequency amplifier. The amplified audio frequency energy is transferred from the output of tube 2, through the audio frequencytransformer 23, to the input of tube I, now functioning as a second stage of; audio frequency amplification. The amplified audio frequency energy of the second audio frequency stage is then transferred f om the utput of tube I to the aforedescribed utilization means,

As tat d h retofore, no seri u di i are encountered in the above type of receiver where normal grid bias is employed and the instantaneous grid swing of tubes and 2 does not run either into the positive region of the tube characteristic, or the cut-off region of the latter. However, with the increase in modern broadcasting power, and also in virtue of the hi h amplifying power of tubes I and 2, the instantaneous grid swing of considerable magnitude results in the grid entering the positive region or the cutoff region of the tube characteristic. This results in distortion and modulation howl.

off point of the characteristic.

. regulation of the volume control action.

entering the positive region.

matic volume control circuit is employed, it will.

be found that the impression of radio frequency voltage and audio frequency voltage upon the grid: of each. of the tubes. preceding the detector will be: of sufficient magnitude to swing. the grid intothe positive region of the tube characteristic, thus resulting in overload howl and modulation distortion, or in temporarily silencing of thereceiver in virtue of the'grid swinging to the cut- The aforementioned defects are successfully solved by the employment of remotecut-off tubes used in combination'with an automatic volumecontrol circuit. The action is as follows: 7

Assuming the receiver tuned to'a strong local station, the normal bias necessary for maximum sensitivity is. automatically increased to effect a reduced amplification as well as to permit a greater instantaneous grid swing with the'grid This is accomplished in virtue of the fact that as the signal strength increases above a predetermined level, the energy transferred from the input of the detector tube 3. to the cathode and anode of diode 33 increases.

This'results in a greater potential drop across the resistor 2|, with the result that the grids of tubes land 2 haveitheir bias increased, it being understood that the biasing arrangements 8 and 8' in the cathode leads of each of tubes I and 2 provide the normal bias necessary for maximum sensitivity. The adjustable tap of lead '20 is adjusted to a point on resistor 2lto provide for Additionally, in virtue of the remote cut-off tube the increased instantaneous grid swings do notenter the cut-off region so that distortion and modulae tion howl do not arise from this, source. That is,

the automatic volume control feature alone does not solve the problem as the increased bias on other than remote cut-offv tubes would run the signal into the cut-offregion where the same difficulty arises. r

.It is thus seen that the normal grid bias which is near the plus region and which is necessary for maximum gain, is automatically set back and adjusted to accommodate the greater swings of stronger stations. It is the combination of automatic volume control and the remote cut oif tube that admirably lends itself to the difficulties in duplexing encountered with the great amplification'. of modern amplifiers and the increased strength of, modern broadcast transmitters. Hence, it will be seen that the present invention for the first time provides in a reflex receiver, and specifically in an inverse duplex receiver, an arrangement whereby volume control of the receiver can be effected by means of grid bias control of the tubes preceding the detector. This has not been done in reflex receivers of the prior art for the reason that signals of even moderate tive bias were increased to reduce the gain.

V Additionally, it is to be .noted that the present invention provides in reflex receivers an automatic volume control circuit which functions in a truly uniform manner over substantially the to any desired type of utilization means.

entire range of receiver radio: frequency input. Fig. 35 graphically'demonstrates. this. advantage. To explain this point more fully it is pointed out, that the increase in bias created by'the' auto matic control function is effective inureducing both radio and audio amplification. The audio .gain occurring after. the volume. control tube may be over-controlled which will offset the lack of perfectly flat control in radio frequency circuits due to the presence of the control tube following the radio amplification.

In Fig.2 there is shown a. reflex receiver employing a modified form of the invention. The circuit elements are substantially similar tov those shown in Fig. 1 with the exception. that but a single stageof audio frequency amplification is employed. That is to say, after amplification in two tuned stages of radio frequency amplification,

and subsequent to detection, the detected output of tube 3 is impressed upon the input of tube I through an audio frequency transformer 50', the energy being transferred through a primary path which includes the lead and the primary 52. The secondary path of the audio frequency coupling includes the lead 53, the secondary 54., and the lead 55' adjustably connected to. a predetermined point on the resistor 2|.

It willbe noted that an audio frequenoytransformer 55 has its primary connected in the anode circuit of tube .I, while its secondary is connected Furthermore, i-t is to be noted that'while detection in Fig.1 is accomplished by grid rectification, in Fig. '2 anode rectification is employed for detection, bias being provided for the grid circuit of detector tube 3 by the well known biasing arrangement 5! disposed in the grounded cathode lead of the tube 3. A radio frequency choke '58 is connected in series in the anode circuit screen grids are again employed for securing the remote cut-off characteristic, it being pointed out that the portion 68 of each of the screen grids of tubes l and 2 denotes a portion of the screen grid which is of different pitch than the remainder 'of the screen grid. This type of va- 7 riable mu tube is clearly shown in the aforementioned Ballantine and Snow publication and need not be described in any further details in this application. 7

The operation of the receiver shown in Fig. 2 is identical with that shown in Fig. 1. additionally, demonstrates an added advantage of the present invention 'in that it shows that the present invention can be employed with but a single stage of audio frequency amplification,.

the adjustable tap of the lead being set at such a point on the resistor 2| that the uniform gain control action denoted by the full line in Fig. 3 is secured. The reason for this is as follows: The amount of automatic grid voltage control can be adjusted to affect the initial control of volume. Then,.as louder signals are impressed, the audio amplification isovercontrolled as this gain occurs after the control tube, as discussed in connection with Fig. 1. r

The'present invention is particularly useful in automobile radio receivers, in airplane radio re- Fig. 2,

radio receivers.

"frequency variations and undercontrolling the receiver the reduced power results in a cheaper power pack. Of course, the reduced number of tubes also results in a saving in tubes alone, but the power consideration is the most important.

While I have indicated and described several systems for carrying my invention into effect,

it will be apparent to one skilled in the art thatmy invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In a system comprising a plurality of variable mu amplifier tubes arranged for interstage operation, and circuit connections for said amplifiers, said circuit connections being so arranged that said amplifiers are operable to amplify current variations of two different bands of frequencies, and means including said circuit connections whereby the current variations within one band of frequencies are impressed upon the amplifiers in a predetermined order, and the current variations within the other band of frequencies are impressed upon the same amplifiers in a predetermined order, and means, responsive to variations in carrier energy magnitude of the higher of said bands of frequencies, for automatically controlling the gain of said amplifiers in such a manner that the amplification of said higher band is undercontrolled and the lower band is overcontrolled.

2. A multi-stage amplification system comprising a plurality of amplifier tubes, said tubes having remote cut-off plate current-grid voltage characteristics, circuit connections for said amplifier tubes, the arrangement being such that superaudible high frequency current variations are repeatedly amplified by said tubes in a predetermined order, and means whereby a plurality of said tubes are rendered operable to repeatedly amplify audible frequency current variations in a predetermined order, and a bias control circuit connected with the grid circuits of said latter tubes for automatically controlling the gain thereof in a sense such that the amplification of the superaudible frequency currents is undercontrolled and that of the audible frequency currents is overcontrolled when the magnitude of the high frequency current variations increases above a predetermined level.

3. A multi-stage amplification system comprising a plurality of variable mu amplifier tubes, circuit connections for said tubes, the arrangement being such that modulated high frequency current variations are repeatedly amplified by said tubes in a predetermined order, means for rectifying the amplified output of said amplification system whereby lower frequency current variations are derived, means whereby a plurality of said amplifier tubes are rendered operable to repeatedly amplify the lower frequency current variations from said rectifying means, and an automatic gain control circuit, connected between said rectifying means and said last named plurality of tubes in a manner to control the gain of the tubes in a successively decreasing sense, for overcontrolling the amplification of the lower amplification of the high frequency variations.

4. A multi-stage amplification system comprising a plurality of amplifier tubes constructed to have variable amplification constants, the tubes being arranged to repeatedly. amplify modulated radio frequency current variations in a predetermined order, means inter-connecting said amplifier tubes operable to transmit radio frequency currents with relatively high efficiency but inoperable to transmit low frequency currents to an appreciable extent, means for rectifying the amplified radio frequency output of said amplification system whereby low frequency current variations are derived, means whereby a plurality of said amplifier tubes are rendered operable to repeatedly amplify the low frequency current variations from said rectifying means in the in verse order as compared with the aforementioned predetermined order, said last mentioned means being operable to transmit low frequency current variations with relatively high efiiciency but inoperable to transmit radio frequency current variations to an appreciable extent,-and an automatic gain control circuit, connected between the input of said rectifying means and the input circuits of each of said amplifier tubes which are rendered operable to amplify the low frequency current variations in a manner to control the gain of said last tubes in a successively decreasing sense for regulating the amplification of the radio and low frequency currents in sufficiently different senses to maintain the overall amplification of the system uniform.

5. A receiving system comprising at least one variable mu amplifier tube, a succeeding detector stage, circuit connections between said amplifier tube and said detector stage, the arrangement being such that high frequency current variations are amplified by said amplifier tube, and means whereby said tube is rendered operable to am plify the low frequency output of said detector stage whereby both the high frequency and low frequency current variations are simultaneously amplified by said amplifier tube, and an automatic gain control circuit connected between said detector stage and the input of said amplifier tube for preventing the instantaneous grid swing of said amplifier tube fromentering the positive region of the amplifier tube characteristic, the variable mu construction of said amplifier tube preventing the instantaneous grid swing from entering the cut-off region of the amplifier tube characteristic.

6. The method of utilizing electron discharge tubes having remote cut-off plate current-grid voltage characteristics as amplifiers, which consists in utilizing the tubes in a predetermined sequence for repeatedly amplifying high frequency variations, rectifying the amplified high frequency variations thereby obtaining lower frequency variations, subsequently utilizing the same tubes in inverse sequence for repeatedly amplifying the lower frequency variations, and automatically controlling the gain of said amplifiers in such a manner that the instantaneous grid swings of said amplifiers are prevented from entering the positive region of their characteristics, said remote cut-01f characteristics preventing the instantaneous grid swings from entering the cutoff region thereof when said automatic control action is effective.

7. In a radio receiver of the reflex type which includes a radio frequency amplifier, a detector and connections between the detector output and l .the amplifier input for impressing #deteoted currents on the amplifier whereby radio f-reguency and detected currents are {simultaneously amplified in said amplifier, means for preventing over,- load howl and modulation distortion or-cut-ofi-in theamplifierzdue to the reception of astrong local station, said means comprising an automatic V the amplifier having :a grid -voltage-plate current characteristic such'that it has a substantially -r.e-

mote .negati-vebut-off point.;

The method of receiving broadcast signals which consists in amplifying rcollected signals :-of a desired frequency in a radio-frequency ampli- ,fier having-an exponential relation between-anodelcurrent and control grid potential, detecting the amplified signals, impressing the audio component of thedetect'ed signals upon said amplifier for audio vamplification -of-said component, rectifying said amplified signals, and regulating the gain of said amplifier 'with the direct current Pcomponentof' the rectified signal energy in such a manner that :the amplification of the undetected signals and the audio component of :detected signals are diiferently-controlled but insuch'asense ,that the intensity level of the-signals detected is substantially uniform.

'9. In combinationwith a radio receiver of the type having a high' frequency amplifier, a detec- .tor, and connections between the detector output :circuitx-and the amplifier input circuit whereby :the amplifier additionally amplifiesthe audio fre- :quency component of the detected high ire:- "quency energy, said amplifier including a'tube having an exponential relation between anode current and control grid potential, a diode :rectifier connected to the :amplifier output circuit,

' and .a direct current connection between a :se

ilected point in the diode circuit and the input circuit 'of said amplifier, said point being such that the amplification of the :audio component of detected energy is .sufiiciently overcontrolled V with respect to the amplification of the undetectedenergy that a uniform gain control for the receiver :is secured. 7

10. In combination with a radio receiver ofthe type having a :high frequency amplifier, -a detector, and connections between the detectorioutput circuit and the amplifier input circuit whereby the amplifier additional-lygamplifies the audio frequency component of the-detected high'frequency energy, .-.said amplifier including a tube having an exponential relation between anode current and control grid potential, a diode rectifier connected to :the amplifier output circuit and a direct cmrent connection between a selected point in the V diode circuit and the input circuit of said ampli- .fier, said diode rectifier preventing the control grid of the amplifier tube from drawing grid current, .and said relation preventing the amplifier :tube 'fromcut-ting off with wide control grid voltage swings. V DAVID GRIMES. 

